doc: issue2551415 correct doc for change input->input_payload in 2.5 the rest interface changed a variable name from input to input_payload. An earlier commit changed the rest docs. This commit adds an item for it to the upgrading 2.4.0->2.5.0 section. Also cross reference added to the rest docs with the updated examples.

#!/usr/bin/env python3
import sys
import roundup

from roundup.demo import main

sys.exit(main())